Thermoelectric Generators (TEG) can be used on automobiles to harvest energy from exhaust waste heat. Besides the improvement on material side, the optimization of geometry of the module is also important to maximize the output power density and will be addressed in this paper. A thermal resistance network is established based on heat conduction and radiation from heat source to heat sink. Although the heat transfer model is based on cylindrical exhaust pipe geometry, the thermo-element is approximated as plane geometry because the ceramic layer and thermoelectric layer are much smaller compared with the exhaust pipe diameter. The TE material we proposed to recover waste heat energy is magnesium silicide (Mg2Si), which has a reasonable figure of merit in the automobile exhaust temperature range, and the process is thermal spray compatible, which is a mass productive method currently under investigation. Another material that used for comparison is titanium oxide. Based on the Seebeck coefficient, thermal and electrical conductivity of our thermal sprayed samples, the thermoelectric leg length and the area ratio between thermoelectric element and total module area are optimized for maximum power density output. The optimal leg length is around 0.85mm, and the air gap is as small as possible. A parameter sensitivity analysis is conducted to investigate the influence of ceramic layer thickness, exhaust pipe radius, electrical contact resistance, hot and cold side temperature, Seebeck and electrical conductivity on the optimal leg length.

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